Thermostat with failure diagnosis function and mehod for failure diagnosis of thermostat using the same

ABSTRACT

Disclosed are a thermostat with a failure diagnosis function and methods for diagnosing a failure in a thermostat. The thermostat is to be installed between an engine and a radiator to control cooling water to bypass back to the engine or to circulate to the radiator by means of a valve operated depending on temperatures in the cooling water circulated between the engine and the radiator. The thermostat may include a plurality of water temperature sensors installed before and after the valve to measure the temperatures in the cooling water before and after the valve, for determining whether or not the valve is normally operated by comparing a difference of the temperatures in the cooling water before and after the valve with a preset temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of Korean Patent Application Number 10-2013-0162020 filed on Dec. 24, 2013, the entire contents of which application are incorporated herein for all purposes by this reference.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a thermostat for controlling circulation of cooling water in a vehicle, and, particularly, to a thermostat with a failure diagnosis function, capable of determining whether or not the thermostat is normal by respectively measuring temperature in cooling water before and after a valve of the thermostat, and a method for failure diagnosis of a thermostat using the same.

2. Description of Related Art

In a vehicle equipped with an engine, cooling water circulates the engine and a radiator for cooling of the engine and a thermostat which controls the flow of cooling water depending upon temperature of the cooling water is installed between the engine and the radiator.

For example, as shown in FIG. 1, cooling water discharged from an engine 1 is bypassed from the thermostat 110 immediately to the engine 1 until the engine 1 is warmed up so that the engine 1 is warmed up rapidly. Meanwhile, the thermostat 110 allows cooling water to be cooled via a radiator 2 in a state in which the engine 1 is warmed up and the cooled water is supplied to the engine 1, so that the engine 1 operates at a proper temperature. A water pump 2 serves to circulate cooling water.

The thermostat 110 is provided therein with a valve operating according to temperature and the valve 12 is opened and closed according to the temperature of cooling water so as to control a flow direction of the cooling water.

Meanwhile, in connection with reinforced exhaust gas regulation of a vehicle recently, monitoring for a cooling system is also a regulation object.

In order to meet such regulation in the related art, it is determined that the engine 1 is sufficiently warmed up when the sum of fuel injection quantity reaches a certain amount after starting the vehicle and then it is determined that the thermostat fails when the temperature of cooling water is equal to or less than a certain temperature. That is, when the temperature of cooling water is equal to or less than a given temperature despite that the sum of fuel injection quantity exceeds a certain value and thus the engine should be warmed up, it is determined that the thermostat 110 becomes a so-called “closed-failing” which is not transitioned from an opened state to a closed state such that cooling water is not circulated to the radiator 2.

However, there is a problem in that a method for failure diagnosis using the conventional thermostat may determine only closed-failing of the thermostat 110 whereas may not determine “open-failing” in which the valve of the thermostat is not opened.

In addition, since it is determined whether or not the thermostat is normal according to an amount of fuel injection of the engine 1 and a cooling water temperature within the thermostat 110, there is a problem in that significant test costs and long development period are required for setting a temperature map to be applied to a control method. That is, since all operating times of the thermostat differ from each other according to atmospheric temperature and vehicle speed and thus the operating times must be established as a map through repeated tests, significant test costs and long development period are required.

Moreover, the determination whether or not the thermostat is normal may be very inaccurate. In addition, the temperature map for determining whether or not the thermostat 110 is normal is configured as a function of only atmospheric temperature and vehicle speed and does not apply conditions such as loading of goods, climbing, and solar radiation, so that the determination whether or not the thermostat is normal may be very inaccurate.

The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

SUMMARY OF INVENTION

The present invention is directed to a thermostat with a failure diagnosis function, capable of diagnosing a failure within the thermostat regardless of atmospheric temperature and operating conditions by measuring water temperature before and after a valve of the thermostat through water temperature sensors respectively installed before and after the valve of the thermostat, and a method for failure diagnosis of a thermostat using the same.

According to various aspects of the present invention, a thermostat with a failure diagnosis function, wherein the thermostat is to be installed between an engine and a radiator to control cooling water to bypass back to the engine or to circulate to the radiator by means of a valve operated depending on temperatures in the cooling water circulated between the engine and the radiator, the thermostat including a plurality of water temperature sensors installed before and after the valve to measure the temperatures in the cooling water before and after the valve, for determining whether or not the valve is normally operated by comparing a difference of the temperatures in the cooling water before and after the valve with a preset temperature.

If a temperature in the cooling water introduced from the engine to the thermostat is lower than a closed completion temperature at which the valve is set to be fully closed and the difference of the temperatures in the cooling water before and after the valve is lower than a preset closed-failing determination temperature, the valve may be determined to be closed-failing which is not closed despite of being closed.

If the temperature in the cooling water introduced from the engine to the thermostat is higher than an open completion temperature at which the valve is set to be opened and the difference of the temperatures in the cooling water before and after the valve is higher than a preset open-failing determination temperature, the valve may be determined to be open-failing which is not opened despite of being opened.

The closed-failing determination temperature or the open-failing determination temperature may be set as about 10° C.

The water temperature sensors may include a first water temperature sensor installed at a side to which the cooling water is introduced from the engine to the thermostat and a second water temperature sensor installed at a side from which the cooling water passing through the valve is discharged to the radiator.

According to various other aspects of the present invention, a method for diagnosing a failure in a thermostat, wherein the thermostat is to be installed between an engine and a radiator to control cooling water to bypass back to the engine or to circulate to the radiator by means of a valve operated depending on temperatures in the cooling water circulated between the engine and the radiator and measured by water temperature sensors installed before and after the valve, the method including: performing closed completion estimation by comparing a temperature in the cooling water at a side to which the cooling water is introduced from the engine with a closed completion temperature at which the valve of the thermostat is set to be fully closed, performing closed-failing determination in which, if the temperature in the cooling water introduced from the engine is lower than the closed completion temperature at the performing of the closed completion estimation, a difference of the temperatures in the cooling water before and after the valve is compared with a closed-failing determination temperature, and performing closed-failing confirmation in which, if the difference of the temperatures in the cooling water before and after the valve is lower than the closed-failing determination temperature at the performing of the closed-failing determination, the valve is confirmed to be closed-failing which is not closed despite of being closed.

The method may further include, prior to the performing of the closed completion estimation, performing diagnosis beginning determination to determine whether or not the temperature in the cooling water introduced from the engine is higher than a preset diagnosis beginning temperature, wherein if the temperature in the cooling water introduced from the engine is higher than the diagnosis beginning temperature, the performing of the closed completion estimation is carried out.

The diagnosis beginning temperature may be set to be higher than an atmospheric temperature by a preset temperature. The closed completion temperature may be set to be lower than a closed beginning temperature at which the valve is set to be closed by a preset temperature.

The method may further include performing normally closed confirmation in which, if the temperature in the cooling water introduced from the engine is not lower than the closed completion temperature at the performing of the closed completion estimation, the valve is confirmed to be normally closed when having to be closed. The method may further include performing normally closed confirmation in which, if the difference of the temperatures in the cooling water before and after the valve is not lower than the closed-failing determination temperature at the performing of the closed-failing determination, the valve is confirmed to be normally closed when having to be closed.

According to still various other aspects of the present invention, a method for diagnosing a failure in a thermostat, wherein the thermostat is to be installed between an engine and a radiator to control cooling water to bypass back to the engine or to circulate to the radiator by means of a valve operated depending on temperatures in the cooling water circulated between the engine and the radiator and measured by water temperature sensors installed before and after the valve, the method including: performing open completion estimation by comparing a temperature in the cooling water at a side to which the cooling water is introduced from the engine with an open completion temperature at which the valve of the thermostat is set to be fully opened, performing open-failing determination in which, if the temperature in the cooling water introduced from the engine is higher than the open completion temperature at the performing of the open completion estimation, a difference of the temperatures in the cooling water before and after the valve is compared with an open-failing determination temperature, and performing open-failing confirmation in which, if the difference of the temperatures in the cooling water before and after the valve is higher than the open-failing determination temperature at the performing of the open-failing determination, the valve is confirmed to be open-failing which is not opened despite of being opened.

The method may further include performing normally open confirmation in which, if the temperature in the cooling water introduced from the engine is not higher than the open completion temperature at the performing of the open completion estimation, the valve is confirmed to be normally opened when having to be opened. The method may further include performing normally open confirmation in which, if the difference of the temperatures in the cooling water before and after the valve is not higher than the open-failing determination temperature at the performing open-failing determination, the valve is confirmed to be normally opened when having to be opened.

The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrams schematically illustrating a state in which circulation of cooling water is controlled by a thermostat in a typical engine.

FIG. 3 is a perspective view illustrating an exemplary thermostat with a failure diagnosis function according to the present invention.

FIG. 4 is a cross-sectional view taken along line A-A in FIG. 3.

FIG. 5 is a flowchart illustrating an exemplary method for diagnosing a failure in a thermostat according to the present invention.

FIG. 6 is a flowchart illustrating another exemplary method for diagnosing a failure in a thermostat according to the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

In a thermostat with a failure diagnosis function according to various embodiments of the present invention, respective water temperature sensors 21 and 22 are installed before and after a valve 12 to control circulation of cooling water in the thermostat 10 and it is determined whether or not the valve 12 is normally operated using a temperature difference measured by the water temperature sensors 21 and 22.

In the thermostat 10 as shown in FIGS. 3 and 4, the valve 12 operated depending upon temperature is installed inside a main body 11 equipped with a plurality of ports. The thermostat 10 is formed with a bypass port 11 b, an Exhaust Gas Recirculation (EGR) port 11 d, and a heater core port 11 e at a side to which cooling water is introduced from an engine 1 through an engine connection port 11 a on the basis of the valve 12. A radiator port 11 c connected to a radiator 2 is then formed at a side opposite to the engine connection port 11 a on the basis of the valve 12.

Accordingly, cooling water introduced into the thermostat 10 is bypassed through the bypass port 11 b, is supplied through the EGR port 11 d to an EGR unit, or flows through the heater core port 11 e to a heater core. In addition, when the valve 12 is opened by increase of temperature in cooling water, the cooling water is supplied through the radiator port 11 c to the radiator.

In the thermostats 10, the respective water temperature sensors 21 and 22 are installed before and after (e.g., upstream and downstream of) the valve 12 and it is determined whether or not the valve 12 is normally operated by comparing a difference in water temperature before and after the valve 12, which is measured by the water temperature sensors 21 and 22.

Here, the water temperature sensor installed at a side to which cooling water is introduced from an engine 1 is referred to as a first water temperature sensor 21 whereas the water temperature sensor installed at a side from which cooling water is discharged to the radiator 2 is referred to as a second water temperature sensor 22, on the basis of the valve 12, and a description will be given of. Accordingly, cooling water introduced from the engine 1 passes through the valve 12 via the first water temperature sensor 21 and is then supplied to the radiator 2 via the second water temperature sensor 22.

When the temperature of cooling water before and after the valve, which is measured by the first and second water temperature sensors 21 and 22 installed before and after the valve 12 in the thermostat 10, is input to an Electronic Control Unit (ECU) 31, the ECU 31 diagnoses whether or not the valve 12 is normally operated. In addition, the ECU 31 may also consider other conditions such as atmospheric temperature, climbing, and loading of goods.

Depending upon operating conditions of a vehicle, the valve 12 of the thermostat 10 may be normal when closed (for instance, before warming up) whereas the valve 12 of the thermostat 10 may be normal when opened (for instance, after warming up).

Accordingly, the ECU 31 determines whether the valve 12 is normal when closed or is normal when opened depending upon the operating conditions of the vehicle, using a temperature T1 of cooling water introduced from the engine 1, which is measured by the first water temperature sensor 21. The ECU 31 determines a difference in temperature before and after the valve 12, namely, a difference between a temperature T1 of the first water temperature sensor 21 and a temperature T2 of the second water temperature sensor 22. Consequently, when the temperature difference is lower than a preset temperature difference, the valve 12 is opened so that cooling water flows, whereas when the temperature difference is higher than a preset another temperature difference, the valve 12 is closed so that cooling water does not flow.

Therefore, it is determined whether the valve 12 is normal when opened or is normal when closed, based on the temperature in cooling water measured by the first water temperature sensor 21, and the determined result is compared with an actual opening and closing state of the valve 12, with the consequence that it is determined whether or not the valve 12 is normally operated.

For example, when the temperature T1 in cooling water introduced from the engine 1 to the thermostat 10 is lower than a closed completion temperature Tc at which the valve 12 is set to be fully closed (T1<Tc) and the temperature difference T1−T2 before and after the valve 12 is lower than a preset closed-failing determination temperature Tcf ((T1−T2)<Tcf), the valve 12 may be determined to be closed-failing which is not closed despite of being closed.

On the other hand, when the temperature T1 in cooling water introduced from the engine 1 to the thermostat 10 is higher than an open completion temperature To at which the valve 12 is set to be fully opened (T1>To) and the temperature difference T1−T2 before and after the valve 12 is higher than a preset open-failing determination temperature Tof ((T1−T2)>Tof), the valve 12 may be determined to be open-failing which is not opened despite of being opened.

Here, the closed-failing determination temperature Tcf or the open-failing determination temperature Tof may be set, for example, as 10° C. or about 10° C.

The thermostat with a failure diagnosis function is controlled according to a method for failure diagnosis of a thermostat to be described later, so that it is diagnosed whether or not the thermostat is normal.

Hereinafter, a method for failure diagnosis of a thermostat according to various embodiments of the present invention will be described with reference to FIG. 4. In accordance with the method for failure diagnosis of a thermostat according to the present invention, in a case in which the valve 12 should be closed in the thermostat 10 as before the engine is warmed up, temperatures before and after the valve, which are measured by the first water temperature sensor 21 installed at the side to which cooling water is introduced from the engine 1 in the thermostat 10 and the second water temperature sensor 22 installed at the side from which cooling water is discharged to the radiator 2 via the valve 12, on the basis of the valve 12 in the thermostat 10, are compared with each other, and thus it may be possible to diagnose closed-failing in which the valve 12 is not closed despite of being closed. That is, the temperature T1 in cooling water introduced from engine 1 is measured by the first water temperature sensor 21 and the temperature T2 in cooling water flowing to the radiator via the valve 12 is measured by the second water temperature sensor 22.

To this end, the method for failure diagnosis of a thermostat includes a closed completion estimation step S120 comparing a temperature in cooling water at a side to which cooling water is introduced from the engine 1 with a closed completion temperature Tc at which the valve of the thermostat 10 is set to be fully closed, a closed-failing determination step S130 in which when the temperature T1 in cooling water introduced from the engine is lower than the closed completion temperature Tc (T1<Tc) at the closed completion estimation step S120, the temperature difference T1−T2 of cooling water before and after the valve 12 is compared with a closed-failing determination temperature Tcf, and a closed-failing confirmation step S140 in which when the temperature difference T1−T2 before and after the valve 12 is lower than the closed-failing determination temperature Tcf ((T1−T2)<Tcf) at the closed-failing determination step S130, the valve 12 is confirmed to be closed-failing which is not closed despite of being closed.

A diagnosis beginning determination step S110 determines whether or not the temperature in cooling water introduced from the engine 1 is higher than a preset diagnosis beginning temperature Td. It is determined whether or not the temperature in cooling water introduced from the engine 1 is higher than the diagnosis beginning temperature Td at the diagnosis beginning determination step S110, and thus the following processes are performed only when the temperature T1 in cooling water measured by the first water temperature sensor 21, namely, the temperature in cooling water introduced from the engine 1 is higher than the diagnosis beginning temperature Td. Here, the diagnosis beginning temperature Td is preferably set to be higher than the atmospheric temperature by a preset temperature. For example, the diagnosis beginning temperature Td is set to be higher than the atmospheric temperature such as by 30° C., and it is determined whether or not the temperature in cooling water introduced from the engine to the thermostat 10 is equal to or more than the diagnosis beginning temperature Td. That is, since the thermostat 10 is operated in a state in which the engine 1 is started and is then warmed up to a certain degree, the following respective steps are performed only when the temperature in cooling water is equal to or more than the diagnosis beginning temperature Td set to be higher than the atmospheric temperature.

The closed completion estimation step S120 compares the temperature in cooling water introduced from the engine 1, which is measured by the first water temperature sensor 21 installed at the side to which cooling water is introduced from the engine 1 in the thermostat 10 with the closed completion temperature Tc at which the valve 12 is set to be fully closed. At the closed completion estimation step S120, it is determined whether or not the valve 12 is closed by comparing the temperature T1 in cooling water introduced from the engine 1 with the closed completion temperature Tc at which the valve 12 is set to be fully closed in the thermostat 10. The closed completion temperature Tc is set to be lower than a closed beginning temperature Tcs at which the valve is set to be closed by a preset temperature. For example, the closed completion temperature Tc may be set to be lower than the closed beginning temperature Tcs such as by 5° C. The closed completion temperature Tc is set to be lower than the closed beginning temperature Tcs at which the valve 12 should be closed. Therefore, when the valve 12 is normal, the valve 12 is in a closed state at the closed completion temperature Tc.

The closed-failing determination step S130 compares a difference between the temperature T1 in cooling water measured by the first water temperature sensor 21 and the temperature T2 in cooling water measured by the second water temperature sensor 22, namely, a temperature difference of cooling water before and after the valve 12 with a preset closed-failing determination temperature Tcf. The closed-failing determination temperature Tcf is a temperature to determine whether or not the valve 12 is closed in a case in which the valve 12 should be closed. For example, the closed-failing determination temperature Tcf may be set as 10° C. or about 10° C.

The closed-failing confirmation step S140 is a step in which when the temperature difference before and after the valve 12 is lower than the closed-failing determination temperature Tcf ((T1−T2)<Tcf) at the closed-failing determination step S130, the valve 12 is confirmed to be closed-failing which is not closed despite of being closed. When the temperature difference measured by the first and second water temperature sensors 21 and 22 respectively installed before and after the valve 12 is lower than the closed-failing determination temperature Tcf, cooling water flows through the valve 12. In this case, the valve 12 is confirmed to be closed-failing since the valve 12 is not closed despite of being closed.

Meanwhile, when the temperature in cooling water introduced from the engine 1 is not lower than the closed completion temperature Tc at the closed completion estimation step S120, a normally closed confirmation step S150 may be performed in which the valve 12 is considered to be normally closed when having to be closed.

In addition, when the temperature difference measured by the first and second water temperature sensors 21 and 22 respectively installed before and after the valve 12 is higher than the closed-failing determination temperature Tcf at the closed-failing determination step S130, cooling water does not flow through the valve 12. In this case, the valve 12 is confirmed to be normal since the valve of the thermostat 10 is closed (S150).

Accordingly, in various embodiments of the present invention, it is determined whether or not the valve 12 is normal, namely, the valve 12 is closed when cooling water is not circulated to the radiator 2 due to insufficient warming up thereof and is bypassed so as to be re-circulated to the engine 1. In a normal state, the valve 12 is closed.

A method for failure diagnosis of a thermostat according to various other embodiments of the present invention is performed for diagnosing whether or not the valve 12 is normally opened. When the engine 1 is sufficiently warmed up, the valve 12 of the thermostat 10 has to be opened in order to circulate cooling water introduced from the engine 1 to the radiator 2. In this case, it is determined whether or not the valve 12 is normally opened using the temperature difference measured by the first and second water temperature sensors 21 and 22 respectively installed before and after the valve 12.

To this end, the method for failure diagnosis of a thermostat according to various other embodiments of the present invention includes an open completion estimation step S220 comparing a temperature T1 in cooling water measured by the first water temperature sensor 21 at a side to which cooling water is introduced from the engine 1 with an open completion temperature To at which the valve of the thermostat 10 is set to be fully opened, an open-failing determination step S230 in which when the temperature T1 in cooling water introduced from the engine is higher than the open completion temperature To (T1>To) at the open completion estimation step S220, the temperature difference of cooling water before and after the valve 12 is compared with an open-failing determination temperature Tof, and an open-failing confirmation step S240 in which when the temperature difference before and after the valve 12 is higher than the open-failing determination temperature Tof at the open-failing determination step S230, the valve 12 is confirmed to be open-failing which is not opened despite of being opened.

The open completion estimation step S220 determines whether or not the temperature measured by the first water temperature sensor 21 is higher than the open completion temperature To. The open completion temperature To is a temperature at which the valve 12 is set to be fully opened such that the engine 1 is warmed up and is then cooled by cooling water circulated to the radiator 2. Accordingly, in order to diagnose whether or not the valve 12 is normally opened, it is first determined whether the temperature in cooling water, which is sufficiently increased, is higher than a temperature at which the valve 12 is set to be opened, namely, the open completion temperature To.

The open-failing determination step S230 is performed when the temperature T1 measured by the first water temperature sensor 21 is higher than the open completion temperature To (T1>To) at the open completion estimation step S220. The open-failing determination step S230 compares a difference in water temperature before and after the valve 12, namely, a difference between the temperature T1 measured by the first water temperature sensor 21 and the temperature T2 measured by the second water temperature sensor 22 with an open-failing determination temperature Tof. When the temperature difference measured by the two water temperature sensors 21 and 22 is higher than the open-failing determination temperature Tof, the valve may be considered to be not opened. Otherwise, the valve 12 may be considered to be opened.

The open-failing confirmation step S240 is a step in which when the difference in water temperature before and after the valve 12 is higher than the open-failing determination temperature Tof ((T1−T2)>Tof) at the open-failing determination step S230, the valve 12 is confirmed to be open-failing which is not opened despite of being opened. That is, in a case in which the temperature difference before and after the valve 12 is higher than the open-failing determination temperature Tof despite that the valve 12 is determined as having to be opened via the open completion estimation step S220, the valve 12 may be considered to be not opened. When the valve 12 is opened, the temperature difference before and after the valve 12 is lowered due to the flow of cooling water through the valve 12. However, the temperature difference before and after the valve 12 being higher than the preset open-failing determination temperature Tof is considered as the valve 12 being not opened.

Accordingly, when the difference in water temperature before and after the valve 12 is higher than the open-failing determination temperature Tof, the valve 12 is confirmed to be not opened despite of being opened.

Meanwhile, when the temperature T1 in cooling water introduced from the engine is not higher than the open completion temperature To at the open completion estimation step S220, a normally open confirmation step S250 may be performed in which the valve 12 is considered to be normally opened when having to be opened.

In addition, when the temperature difference before and after the valve 12 is not higher than the open-failing determination temperature Tof at the open-failing determination step S230, the valve 12 may be confirmed to be normally opened when having to be opened (S250). That is, when the valve 12 is opened, the temperature difference before and after the valve 12 is lowered due to the flow of cooling water. Therefore, when the temperature difference before and after the valve 12 is not higher than the open-failing determination temperature Tof at the open-failing determination step S230, the valve 12 is confirmed to be normally opened.

In accordance with a thermostat with a failure diagnosis function and a method for failure diagnosis of a thermostat using the same according to various embodiments of the present invention, it may be possible to accurately determine whether or not the thermostat is normal by measuring temperature in cooling water before and after a valve through two water temperature sensors respectively installed before and after the valve in the thermostat. In addition, it may be possible to diagnose open-failing in which the valve of the thermostat is not opened despite of being opened, as well as closed-failing in which the valve of the thermostat is not closed. Furthermore, since whether the thermostat is normal may be diagnosed in various operating conditions, it may be possible to meet regulation on the thermostat for cooling water and monitoring for a cooling system.

For convenience in explanation and accurate definition in the appended claims, the terms “higher” or “lower”, “before” or “after”, and etc. are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. 

What is claimed is:
 1. A thermostat with a failure diagnosis function, wherein the thermostat is to be installed between an engine and a radiator to control cooling water to bypass back to the engine or to circulate to the radiator by means of a valve operated depending on temperatures in the cooling water circulated between the engine and the radiator, the thermostat comprising: a plurality of water temperature sensors installed before and after the valve to measure the temperatures in the cooling water before and after the valve, for determining whether or not the valve is normally operated by comparing a difference of the temperatures in the cooling water before and after the valve with a preset temperature.
 2. The thermostat of claim 1, wherein if a temperature in the cooling water introduced from the engine to the thermostat is lower than a closed completion temperature at which the valve is set to be fully closed and the difference of the temperatures in the cooling water before and after the valve is lower than a preset closed-failing determination temperature, the valve is determined to be closed-failing which is not closed despite of being closed.
 3. The thermostat of claim 2, wherein the closed-failing determination temperature is set as about 10° C.
 4. The thermostat of claim 1, wherein if the temperature in the cooling water introduced from the engine to the thermostat is higher than an open completion temperature at which the valve is set to be opened and the difference of the temperatures in the cooling water before and after the valve is higher than a preset open-failing determination temperature, the valve is determined to be open-failing which is not opened despite of being opened.
 5. The thermostat of claim 4, wherein the open-failing determination temperature is set as about 10° C.
 6. The thermostat of claim 1, wherein the water temperature sensors comprise a first water temperature sensor installed at a side to which the cooling water is introduced from the engine to the thermostat and a second water temperature sensor installed at a side from which the cooling water passing through the valve is discharged to the radiator.
 7. A method for diagnosing a failure in a thermostat, wherein the thermostat is to be installed between an engine and a radiator to control cooling water to bypass back to the engine or to circulate to the radiator by means of a valve operated depending on temperatures in the cooling water circulated between the engine and the radiator and measured by water temperature sensors installed before and after the valve, the method comprising: performing closed completion estimation by comparing a temperature in the cooling water at a side to which the cooling water is introduced from the engine with a closed completion temperature at which the valve of the thermostat is set to be fully closed; performing closed-failing determination in which, if the temperature in the cooling water introduced from the engine is lower than the closed completion temperature at the performing of the closed completion estimation, a difference of the temperatures in the cooling water before and after the valve is compared with a closed-failing determination temperature; and performing closed-failing confirmation in which, if the difference of the temperatures in the cooling water before and after the valve is lower than the closed-failing determination temperature at the performing of the closed-failing determination, the valve is confirmed to be closed-failing which is not closed despite of being closed.
 8. The method of claim 7, further comprising: prior to the performing of the closed completion estimation, performing diagnosis beginning determination to determine whether or not the temperature in the cooling water introduced from the engine is higher than a preset diagnosis beginning temperature, wherein if the temperature in the cooling water introduced from the engine is higher than the diagnosis beginning temperature, the performing of the closed completion estimation is carried out.
 9. The method of claim 8, wherein the diagnosis beginning temperature is set to be higher than an atmospheric temperature by a preset temperature.
 10. The method of claim 7, wherein the closed completion temperature is set to be lower than a closed beginning temperature at which the valve is set to be closed by a preset temperature.
 11. The method of claim 7, further comprising: performing normally closed confirmation in which, if the temperature in the cooling water introduced from the engine is not lower than the closed completion temperature at the performing of the closed completion estimation, the valve is confirmed to be normally closed when having to be closed.
 12. The method of claim 7, further comprising: performing normally closed confirmation in which, if the difference of the temperatures in the cooling water before and after the valve is not lower than the closed-failing determination temperature at the performing of the closed-failing determination, the valve is confirmed to be normally closed when having to be closed.
 13. A method for diagnosing a failure in a thermostat, wherein the thermostat is to be installed between an engine and a radiator to control cooling water to bypass back to the engine or to circulate to the radiator by means of a valve operated depending on temperatures in the cooling water circulated between the engine and the radiator and measured by water temperature sensors installed before and after the valve, the method comprising: performing open completion estimation by comparing a temperature in the cooling water at a side to which the cooling water is introduced from the engine with an open completion temperature at which the valve of the thermostat is set to be fully opened; performing open-failing determination in which, if the temperature in the cooling water introduced from the engine is higher than the open completion temperature at the performing of the open completion estimation, a difference of the temperatures in the cooling water before and after the valve is compared with an open-failing determination temperature; and performing open-failing confirmation in which, if the difference of the temperatures in the cooling water before and after the valve is higher than the open-failing determination temperature at the performing of the open-failing determination, the valve is confirmed to be open-failing which is not opened despite of being opened.
 14. The method of claim 13, further comprising: performing normally open confirmation in which, if the temperature in the cooling water introduced from the engine is not higher than the open completion temperature at the performing of the open completion estimation, the valve is confirmed to be normally opened when having to be opened.
 15. The method of claim 13, further comprising: performing normally open confirmation in which, if the difference of the temperatures in the cooling water before and after the valve is not higher than the open-failing determination temperature at the performing open-failing determination, the valve is confirmed to be normally opened when having to be opened. 